Composite materials

ABSTRACT

A metal matrix composite material made of a fiber reinforcement having a coating of gadolinium and gadolinium boride, preferably as discrete layers, each having a thickness of 1 to 6 microns. This composite is made by coating reinforcing fibers with gadolinium using a deposition technique, further coating the fibers with gadolinium boride by a deposition technique and then consolidating the coated fibers into a metal matrix to form the metal matrix composite.

The present invention relates to matrix composite materials and moreparticularly relates to a coating for the materials.

It is known to prepare composites comprising a fiber reinforcement in ametal matrix. The presence of fibers reduces or prevents the propagationof cracks in the material and is an effective method of increasing theelastic strength and toughness of a material. The resultant materialsare characterised by high strength and stiffness and light weight whichmake them useful for fabrication of aircraft structures, deep seapressure vessels and other like applications. Thus, continuous siliconcarbide fibre reinforced metal matrix composite materials offeradvantages over conventional monolithic materials.

However there is a need to fabricate a protective coating or layer forthe fiber to reduce or prevent interfacial chemical or other types ofmicrostructural degradation. The present invention relates to animproved coating for silicon carbide fibres in a metal matrix compositematerial.

Thus according to the present invention there is provided a matrixcomposite material comprising a fibre reinforcement, the fiberreinforcement having a coating comprising gadolinium and gadoliniumboride.

It is desirable that the fiber coating is capable of accommodating thestrains imposed between the fiber and the matrix caused by thedifferences in coefficient of thermal expansion and hence reduce therisk of matrix cracking or coating fracture. Also the coating isdesirably capable of preventing or reducing the transport of reactantmaterial from the matrix to the fiber thereby avoiding chemicaldegradation of the fiber.

The fiber reinforcement may be any particulate, whisker, laminate, shortor continuous fiber but is preferably ceramic and is most preferablysilicon carbide. The matrix of the composite may be any metal containingmaterial but is preferably a metal alloy, an intermetallic compound, ora ceramic. The preferred metal is titanium and may be in the form of atitanium alloy, an intermetallic compound such as titanium aluminide,TiAl or Ti₃ Al, or a ceramic such as an oxide, carbide, boride ornitride. Other suitable materials include nickel containing materials.Intermetallic compounds tend to have good high temperature propertiesbut are less damage tolerant, e.g. may be vulnerable to cracking, andthus are particularly suitable for the coating of the present invention.

The coating preferably comprises a gadolinium layer having a thicknessof 1 to 6 microns and a gadolinium boride layer having a thickness of 1to 6 microns. The gadolinium boride has a composition GdB_(x) where x isin the range 2 to 6. In the case of titanium based metal matrixcomposites, the gadolinium layer is preferably formed adjacent to thefiber.

The coating can be applied to the fibers by a variety of knowntechniques such as electrodeposition, sputtering etc but the preferredtechnique is by use of electron beam deposition.

The invention also includes a method of fabricating a matrix compositematerial comprising the steps of (a) coating SiC fibers with gadoliniumby use of a deposition technique (b) further coating the fibers withgadolinium boride by use of a deposition technique and (c) consolidatingthe coated fibers into a matrix by means of a suitable consolidationtechnique to form the matrix composite.

The deposition technique may be by use of electron beam deposition. Theconsolidation technique may comprise hot pressing. It is preferred thatfor titanium based matrix composites the first coating on the SiC fiberis gadolinium.

The invention will now be described by way of example only and withreference to the accompanying drawing.

The FIGURE shows a diagrammatic drawing of the experimental arrangementfor coating SiC fiber with a duplex layer of gadolinium and gadoliniumboride.

The fiber 1 were mounted in a cylindrical array on a rotatable fibrecarrier 2. The carrier was located above a Temescal CV14 electron-beam(EB) heated double evaporation source 3 and was rotated at 300 r.p.m.The gadolinium source 7 incorporated a rod-feed mechanism 4 to supplygadolinium to replenish the evaporation pool/crucible 5.

The silicon carbide fiber used were BP Sigma fibres of 100 micronsdiameter and they were coated with a layer of gadolinium followed by alayer of gadolinium boride GdB₃. The coatings were applied by physicalvapor deposition using electron beam evaporation using the Temescalevaporator 3. Separate sources were used for the gadolinium and theboron. Shutters (not shown) were used to prevent gadolinium or boronreaching the fiber during condition adjustment times until substantiallysteady state conditions were attained.

During use, the gadolinium was contained in crucible 5. The power wasincreased up to 3 KW at a rate of 1 KW/min. The shutters were thenopened for 50 seconds. After this time there was a two micron coating ofgadolinium on the fiber 1. The shutters were then closed.

The power setting for crucible 5 was reduced to 0.9 KW and the power forcrucible 6 containing the boron was increased to 1.3 KW at a rate of0.05 KW/min. The shutters were then opened for four minutes. After thistime there was a one micron coating of GdB₃ on top of the gadoliniumlayer.

The resultant coated fibers may then be used in a conventionalfabrication technique for producing a matrix composite material.

We claim:
 1. A matrix composite material comprising a fiberreinforcement wherein the fiber reinforcement has a coating comprisinggadolinium and gadolinium boride.
 2. The material according to claim 1in which the coating comprises a gadolinium layer and a gadoliniumboride layer.
 3. The material according to claim 1 in which thegadolinium layer has a thickness of 1 to 6 microns and the gadoliniumboride layer has a thickness of 1 to 6 microns.
 4. The materialaccording to claim 1 in which the gadolinium boride has a compositionGdB_(x) where x is in the range 2 to
 6. 5. The material according toclaim 1 in which the fiber reinforcement is ceramic.
 6. The materialaccording to claim 5 in which the fiber reinforcement is siliconcarbide.
 7. The material according to claim 1 in which the matrix of thecomposite is a metal alloy, an intermetallic compound, or a ceramic. 8.The material according to claim 1 in which the matrix of the compositecontains titanium.
 9. The material according to claim 8 in which thegadolinium is formed adjacent the fiber.
 10. The material according toclaim 1 in which the matrix of the composite contains nickel.
 11. Amethod of fabricating a matrix composite material comprising the stepsof:(a) coating SiC fibers with gadolinium, (b) further coating thefibers with gadolinium boride, and (c) consolidating the coated fibersinto a matrix.
 12. The method according to claim 11 in which thegadolinium is applied by electron beam deposition.
 13. The methodaccording to claim 1 in which the gadolinium boride is applied byelectron beam deposition.
 14. The method according to claim 1, in whichconsolidating the coated fibers into a matrix in step (c) is achieved byhot pressing.